Organic electro-luminescence (OEL) displays can be divided into passive matrix type and active matrix type according to the driving method. The so-called active matrix organic light-emitting displays (AMOLED) is to use the thin film transistor (TFT) and the capacitor to store image signals and control the luminance and gray scale of OLED.
Though the manufacturing cost is lower and the technology is common for passive matrix OLED; however, the resolution of the panel can't be enhanced due to its driving method. Therefore, the size of the applied products is limited less than 5 inches, which is confined to low resolution and small size market. The active matrix driving method needs to be applied for finer and larger screens. The so-called active matrix means to store image signals by capacitors. Thus, the original brightness of pixels can be maintained after scanning. In this way, extreme brightness of OLED is not needed, longer operation life is guaranteed and requirements for high resolution can be achieved. Active matrix OLED can be put into practice by combining OLED and TFT technology, which not only meets stricter requirements for smoothness and resolution on the monitor market, but also reveals the superb features of OLED to the full extent.
For driving technology at present, development of AMOLED has two directions; one is the analog method and the other one is the digital way. The reason why digital driving is developed is because TFT elements with excellent features (e.g. threshold voltage and mobility) can't be produced through the current LTPS process. Nevertheless, the stringent demands for LTPS process are not required for digital driving since the image non-uniformity due to the characteristic variation of TFT elements can be compensated merely through a simple 2T1C driving circuit.
As a result, digital driving technology will play a certain role in the development of AMOLED in the future if shortcomings of digital driving method can be corrected efficiently and the integrated driving system can be established.
For the application of digital driving technology at the moment, time-ratio and area-ratio modulation methods are used for gray scale. Take the U.S. Pat. No. 6,452,341 as an example for time-ratio technology. It is based on the separation structure of a writing time 61 and a display time 62 (Program Display Separation) for the realization of digital driving scheme. As FIG. 6 shows, 1˜N refers to the scan line and 1˜M refers to the display line. For each sub-frame, the writing time 61 is the same, but the display time 62 is T, 2T, 4T, 8T, 16T and 32T in order respectively from SF1 through SF6. Though this approach is easy to implement and the hardware system is less complicated; however, time utility rate is low since the total writing time 61 from sub-frame SF1 through SF6 occupies a certain portion of the frame time.
For instance, refer to the U.S. Pat. No. 6,452,341 as FIG. 7 indicates. The gray scale is achieved by time-ratio modulation and control of the organic electro-luminescence element with a common cathode potential 71 (VH or VL). Thus, when the resolution of the display panel is 176×240 with the scanning frequency of 120 KHz, the writing time 61 of one sub-frame equals to ( 1/120 K)×240=2 ms. Consequently, the total writing time 61 for the six sub-frames SF1˜SF6 will be 12 ms, which occupies 60% of the frame time 20 ms (1 Frame= 1/50 sec). As OLED is not illuminated during the writing time 61, the display time utility rate only achieves 40%, which is quite low.
This flaw is acceptable for small size applications; however, this problem needs to be overcome for large size or higher resolution requirement in the future. To promote application of digital driving technology, certain solutions are required to correct the defect of low time utility rate of the conventional driving scheme—program display separation.
One of the solutions is to increase the operating frequency, including scanning frequency and data shifting frequency, etc. This method has no problems for earlier display system that uses external driving IC; however, the solution of built-in driving circuit LTPS-TFT adopted to cope with the development trend of system-on-glass (SOG) cannot easily support very high frequency operation.
Japan Patent No. 2001-343933 discloses an AMOLED driving circuit. FIG. 8 shows the circuit of each pixel. The driving circuit in every pixel includes a Writing TFT 81, an Erase TFT 82, a Driving TFT 83, a Storage Capacitance 84, a Write Scan Line 85, a Erase Scan Line 86, a Data Line 87, a Supply Line 88, a Organic Electro-luminescence Element 89. The gate of Writing TFT 81 in the driving circuit is connected to Write Scan Line 85 and the gate of Erase TFT 82 is connected to Erase Scan Line 86. Gray scale is achieved by modulating display time ratio of the frame in this patent, which improves the flaw of low time utility rate in the driving structure of program display separation. Whereas, two sets of scan drivers are required in addition to the Data Driver 91. One of them is Write Scan Driver 92 connecting to Write Scan Line 85 for data writing. Another is Erase Scan Driver 93 connecting to Erase Scan Line 86 for data erasing as FIG. 9 shows. In this way, an extra set of scan driver is required, which also increases the module cost of monitors.